The importance of stretching rate in achieving true stress relaxation in the elasto-capillary thinning of dilute solutions

Abstract

This work focuses on inferring the molecular state of the polymer chain required to induce elasto-capillary stress relaxation and the accurate measure of the polymer relaxation time in uniaxial stretching of dilute polymer solutions. This work is facilitated by the discovery that constant velocity applied at early times leads to initial constant extension rate before reaching the Rayleigh-Plateau instability. Such constant rate experiments are used to correlate initial stretching kinematics with the thinning dynamics in the elasto-capillary Regime. We show that there is a minimum initial strain-rate required to induce rate independent elastic effects. Below the minimum extension rate, insufficient stretching of the chain is observed before capillary instability, such that the polymer stress is comparable to the capillary stress at long times and true stress relaxation is not achieved. Above the minimum strain-rate, the chain reaches a critical stretch before instability, such that during the unstable filament thinning the polymer stress is significantly larger than the capillary stress and true stress relaxation is observed. Using a single relaxation mode Oldroyd-B model, we show that the the minimum strain rate leads to a required initial stretch of the chain before reaching the Rayleigh Plateau limit. Along with the accurate measure of relaxation time, this work introduces a characteristic dimensionless group, called the stretchability factor, that can be used to quantitatively compare different materials based on the overall material deformation/kinematic behavior, not just the relaxation time. Overall, these results demonstrate a useful methodology to study the stretching of dilute solutions using a constant velocity stretching scheme.